Internal-combustion engines are formed basically from two principal parts: an engine block (provided with one or more cylinders) and a crankshaft assembly, with which one or more heads are associated. The crankshaft assembly includes pistons, connecting rods and the crankshaft, this assembly being responsible for moving the pistons inside the cylinders of the engine block. The piston is a cylindrical component, usually composed of a metallic base, and comprises one or more rings responsible for providing a sliding seal between the external edge of the piston and the internal wall of the cylinder.
During movement of the piston inside the cylinder, the piston should adhere totally to the walls of the cylinder in such a manner as to prevent the mixture of air and fuel and to prevent the exhaust gases escaping from the combustion chamber to inside the oil pan during compression and combustion and, furthermore, to prevent the oil in the pan being transferred to the inside of the combustion region. Owing to the high level of wear that such a solution would cause, the solution found has been to design a small degree of clearance between the piston and the internal walls of the cylinder, inserting one or more rings around the piston so as to guarantee the necessary insulation.
Usually, the most modern 4-stroke engines use three rings on each piston, two of these rings being compression rings and the other being an oil ring. The two rings placed closest to the piston head are known as compression segments and their purpose is to ensure that there is no leakage of the gaseous mixture to the inside of the pan when the piston is performing the compression movement. The third piston ring is known as the oil ring and its purpose is to scrape excess oil from the wall of the cylinder, thereby controlling the thickness of the oil film.
In general, piston rings are formed by a metallic external surface to which at least one coating layer suitable for entering into contact with the wall of the cylinder is applied.
The function of the coating is extremely important since it seeks to confer on the ring the properties of low sliding wear, high wear resistance, hardness and durability. However, many of these properties are mutually incompatible and an increase in one of them leads to a reduction in performance in terms of another. It should furthermore be noted that the stresses to which the rings are subjected vary along their surface, i.e. the rings are not subject to all the forces uniformly.
Usually, coating layers are applied using ion plating processes, especially physical vapor deposition (PVD), chemical vapor deposition (CVD), or a galvanic plating process.
By means of studies, tests and simulations, the applicant was able to observe that piston rings, specifically compression rings, suffer premature wear in the regions of the ends of the ring.
In this connection, representative prior art is US document US2004056425, which relates to a PVD coating process that guarantees a coating layer of varying thickness, the thickness of the coating layer in the regions of the ends of the ring being 1.5 to 4 times as thick than the coating layer applied over the region outside of the vicinity of the ends of the ring.
The manufacturing process employed involves a variable rotational speed such that the speed will be minimal when the region of the ends of the ring faces the source of coating material, which means that the thickness in the region in the vicinity of the ends of the ring is greater than the coating thickness in the region opposite the ends of the ring. Nevertheless, this process requires very precise, controlled synchronization during said variation in speeds, since, otherwise, the effect of varying the thickness of the coating will not be achieved.
However, an increase in the coating layer in the region in the vicinity of the ends of the ring alone is insufficient to guarantee that the component is more durable. As described in FIG. 2, the thicker the coating layer in the region in the vicinity of the ends of the ring, the greater will be the contact pressure between the ends of the ring and the cylinder of the engine block. Combustion gives rise to high temperatures and an increase in gas pressure, which leads to premature wear that is further exasperated by the situation of a low level of lubrication and contact pressure between the sliding surfaces of the compression ring and this specific region.
It is thus necessary to devise a piston ring and a coating process capable of guaranteeing a varying thickness of the coating layer on the external surface of the ring such that the region in the vicinity of the ends of the ring comprises a coating layer that is thicker than the coating layer in the opposite region in the vicinity of the ends of the ring and, furthermore, with a reduction in the thickness at the ends of the ring, thus providing a piston ring with a high level of durability owing to excellent wear resistance and lower sliding wear overall, with a high level of thickness in that which is the most critical region of the ring, defined as being in the vicinity of the ends.